United recently announced the return of free snacks on its flights starting this month. In a release today, United touted the return of free snacks. "For the customer heading home from a business trip and for the family embarking on a long-planned vacation, we are building a new inflight experience of comfort and familiarity, with complimentary snacks playing a small but important part," Jimmy Samartzis, United's vice president of food services and United Clubs, said in the release.

After the Sept. 11, 2001, terrorist attacks and the subsequent recession, U.S. airlines removed almost ever perk imaginable on domestic flights. Hot meals disappeared along with legroom, blankets and pillows.

"It was really about survival," said Fernand Fernandez, vice president of global marketing at American.

When Continental Airlines stopped giving out free cookies and mini bags of pretzels in 2011—after its merger with United Airlines—it said the move would save $2.5 million a year.

Since then, mergers have created four mega-carriers that control more than 80 percent of the domestic market. They started charging $25 for each checked suitcase. And more seats have been crammed into planes to maximize profits. Now the industry is making record profits.

"What has changed is that the airlines have been able to fix our core business and be able to reinvest in our customers," Fernandez said.

That means new planes, larger overhead bins and for a long time, nicer first-class cabins.

COOKIES, MEALS

Now a bit of that wealth is trickling back to coach.

This month, American will start offering Biscoff cookies or pretzels to passengers flying between New York and San Francisco or Los Angeles. By April, those snacks will expand to all other domestic routes. In May, American will bring back full meal service for coach passengers between Dallas and Hawaii.

"We know that we have customers who select our airline based on price and we're really excited to offer them a product that is superior to choosing an ultra-low cost carrier," Fernandez said.

Delta Air Lines, the other big legacy carrier, never removed snacks, even during bankruptcy.

American also is expanding its complimentary entertainment on domestic flights with in-seat TVs. Passengers will be able to choose from up to 40 movies, 60 TV shows and 300 music albums. Delta has taken a different path, focusing more on entertainment that passengers can stream to their own devices. It now offers about 67 movies and 138 TV shows for free.

"These are token investments in the passenger experience that will not cost airlines a lot of money but are small ways to make passengers a little bit happier," said Henry Harteveldt, the founder of travel consultancy Atmosphere Research Group. "American and United realized: We don't let other airlines have an advantage on price, why let them have one on pretzels."